Meltblowing techniques for forming very small diameter fibers, sometimes referred to as microfibers or meltblown fibers, from thermoplastic resins are well-known in the art. For example, the production of fibers by meltblowing is described in an article entitled "Superfine Thermoplastic Fibers", appearing in Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346. This article describes work done at the Naval Research Laboratories in Washington, D.C. Another publication dealing with meltblowing is Naval Research Laboratory Report 111437, dated Apr. 15, 1954. Generally, meltblowing techniques include heating a thermoplastic fiber-forming resin to a molten state and extruding the molten resin from a die arrangement having a plurality of linearly arranged small diameter capillaries as molten threads. The molten threads exit the die into a high velocity stream of gas, usually air, which is maintained at an elevated temperature, and which serves to attenuate the threads of molten resin to form fibers having a diameter which is less than the diameter of the capillaries of the die arrangement.
U.S. Pat. No. 3,894,241 to Butin, the disclosure of which is hereby incorporated by reference, discloses the manufacture of nonwoven mats by meltblowing and describes, at column 4, line 57, et. seq., the formation of meltblown fibers having diameters of from about 0.5 to about 400 microns by extruding degraded fiber-forming molten thermoplastic polymer resins as molten threads into an attenuating gas stream. Also disclosed is the fact that the diameter of the attenuated fibers will decrease as the gas flow of the attenuating gas through the gas outlets, which are located on either side of the die tip extrusion capillaries, increases. It is also stated that, at low to moderate attenuating gas velocities, the extruded molten threads, even after attenuation by the gas into fibers, remain essentially continuous with little or no fiber breakage and that fibers produced in such an arrangement have diameters of, preferably, from about 8 to 50 microns. Prior to extrusion the fiber-forming thermoplastic polymer resins are subjected to controlled thermal and oxidative degradation at temperatures ranging from about 550 degrees Fahrenheit to about 900 degrees Fahrenheit, that is from about 288 degrees Centigrade to about 482 degrees Centigrade, preferably from about 600 degrees Fahrenheit to 750 degrees Fahrenheit, that is from about 316 degrees Centigrade to about 399 degrees Centigrade, to effect a requisite degradation of the resin which reduces the viscosity of the fiber-forming resin. Typical fiber-forming thermoplastic resins are listed at column 4, line 35 et. seq. and commercially useful resin throughput rates are stated to be from about 0.07 to 5 grams per minute per die extrusion capillary, preferably at least 1 gram per minute per die extrusion capillary.
While degradation of some thermoplastic resins prior to their extrusion may be necessary in order to reduce their viscosity sufficiently to allow their extrusion and attenuation by the high velocity stream of attenuating gas, there is a limit to the degree of degradation prior to extrusion which can be imposed on a given resin without adversely affecting the properties of the extruded product. For example, excessive degradation of polymeric elastomeric polystyrene/poly(ethylene-butylene)/polystyrene block copolymer resins, may result in the formation of a non-elastic resin. It is believed that the degraded material is non-elastic because the block copolymer resin degrades to form a di-block copolymer resin. Other dangers may be associated with high degradation temperatures. For example, Technical Bulletins SC: 38-82 and SC: 39-85 of The Shell Chemical Company of Houston, Tex., in describing polystyrene/poly(ethylene-butylene)/polystyrene elastomeric block copolymer resins sold by it under the trademark KRATON state that, with respect to the KRATON G 1650 and KRATON G 1652, both of which are block copolymer resins, compounding temperatures of the resin should not be allowed to exceed 525 degrees Fahrenheit, that is 274 degrees Centigrade and that a fire watch should be maintained if the temperature of the resins reaches 475 degrees Fahrenheit, that is 246 degrees Centigrade. With respect to the KRATON GX 1657 block copolymer resin, Shell Technical Bulletin SC: 607-84 gives a warning not to allow the temperature of the block copolymer resin to exceed 450 degrees Fahrenheit, that is 232 degrees Centigrade, and to maintain a fire watch should that temperature be reached. Shell Material Safety Data Sheet designated as MSDS number 2,136 states, with respect to KRATON G-1657 thermoplastic rubber, that the processing temperature of the material should not be allowed to exceed 550 degrees Fahrenheit and that a fire watch should be maintained if that temperature is reached. A Shell Material Safety Data Sheet designated as MSDS 2,031-1 states, with respect to KRATON G-1652 thermoplastic rubber, that the processing temperature of the material should not exceed 550 degrees Fahrenheit and that a fire watch should be maintained if that temperature is reached. Shell Chemical Company Technical Bulletins SC: 68-85 "KRATON Thermoplastic Rubber" and SC: 72-85 "Solution Behavior of KRATON Thermoplastic Rubbers" give detailed information concerning various thermoplastic block copolymer resins which may be obtained from Shell under the trade designation KRATON. The KRATON thermoplastic resins are stated by Shell to be A-B-A block copolymers in which the "A" endblocks are polystyrene and the "B" midblock is, in KRATON G resins, poly(ethylene-butylene) or, in KRATON D resins, either polyisoprene or polybutadiene.
Shell Chemical Company Technical Bulletin SC: 198-83, at page 19, gives examples of commercially available resins and plasticizers useable with KRATON rubber resins. The Bulletin distinguishes between rubber phase, B midblock, associating materials and polystyrene phase, A endblock, associating materials. Among the rubber phase associating materials is a group of resins which are identified as "Polymerized Mixed Olefin" and a plasticizer identified as "Wingtrack 10" having a chemical base of "mixed olefin".
For quite some time those in the art have been attempting to form elastomeric resins into fibrous nonwoven elastomeric webs. In fact, the prior art reveals that experimentation with KRATON G 1650 and KRATON G 1652 brand materials has occurred. For example, U.S. Pat. No. 4,323,534 to des Marais discloses that it was concluded by those in the art that the KRATON G rubber resins are too viscous to be extruded alone without substantial melt fracture of the product. However, des Marais does disclose a process which utilizes blended KRATON G 1650 and KRATON G 1652 resins in the formation of fibrous nonwoven webs and films. In order to overcome the stated viscosity problem the KRATON G 1650 block or KRATON G 1652 copolymer resin was blended with about 20 percent to 50 percent, by weight, of a fatty chemical such as stearic acid prior to extrusion and meltblowing. An extrusion temperature range of 400 to 460 degrees Fahrenheit is disclosed at column 8, line 64 et. seq. and this temperature range is generally within that recommended by the above-mentioned Shell Chemical Company technical bulletins. Unfortunately, the physical properties of the product obtained by this process, for example, a nonwoven mat of meltblown fibers, were apparently unsatisfactory because, after formation of the nonwoven web, substantially all the fatty chemical is leached out of the nonwoven web of extruded microfibers by soaking the web in alcohols having a good ability to solubilize the fatty chemical utilized. In one embodiment, discussed at column 3, lines 8 and 9, the thermoplastic rubber resin is an A-B-A' block copolymer wherein B is poly(ethylene-butylene) and A and A' are selected from the group including polystyrene and poly(alpha-methylstyrene).
U.S. Pat. No. 4,305,990 to Kelly discloses that A-B-A block copolymers having a polybutadiene or polyisoprene midblock and polystyrene endblocks may be extruded as films when blended with an amount of amorphous polyproylene sufficient to enhance the processability of the blend. It is stated in the abstract that the films retain their elastomeric properties and are significantly more processable owing to the presence of the amorphous polypropylene.
Another patent apparently dealing with subject matter stemming from or at least related to the subject matter disclosed in des Marais in U.S. Pat. No. 4,355,425 to Jones which discloses an undergarment that may be made of a fiber formed by meltblowing a blend of a KRATON G rubber with stearic acid. The examples are apparently limited to KRATON G-1652 block copolymers. An extrudable composition, which is stated to be particularly useful at column 4, line 24 et. seq., is a blend of KRATON G 1652 rubber and 20 percent by weight stearic acid as well as minor amounts of other materials. An extrusion temperature of 390 degrees Fahrenheit for the blend of KRATON G 1652 rubber and stearic acid, which is disclosed at column 5, lines 14 and 19, is within the temperature range set forth in the above-mentioned Shell Chemical Company technical bulletins. It is further stated that fibers for making the material can be meltblown as taught in U.S. Pat. No. 3,825,380, to Harding which is said to disclose a die configuration suited for meltblowing the fibers. It should also be noted that the procedures of Jones, as was the case with the procedure of des Marais, indicate the desirability of leaching out the fatty chemical after formation of a fibrous nonwoven web or film from the blend of fatty chemical and KRATON G. See, for example, column 5, lines 60 et seq.